The invention is in the field of timing of operations of moving workpieces in an assembly line process. In a specific embodiment the invention is concerned with operations performed on sheets of paper, such as applying dots or lines of glue, perforations, or slits to the paper. In accordance with the invention the control of the positioning of these operations is made more precise and waste of initial sheets is avoided.
In manufacturing operations on moving workpieces carried on a conveyor, it is important that the operation be carried out at the proper location on the conveyed product, and this can be affected by compensation times or lag times of the operating devices. Such a compensation time is the delay between the time an electrical pulse is sent to the operating device and the time the operation actually takes place, and similarly, the delay to discontinue an action such as applying a slit or glue line. In many systems the timing of the operation, to allow for the compensation time, has simply assumed a fixed time duration between a sheet trip point and the operation of the device, which assumes a constant speed of the conveyor. More accurate encoder-driven systems base operation of the device as a function of position, but still base the lead time for operations on a selected conveyor speed. If the conveyor speed changes in such prior systems, or if an interruption occurs, the resulting position on the workpiece where the operation actually takes place will be shifted. If the operator decides to select a different conveyor speed, he will have to change the lead times for turning "on" and "off" each operating device.
One example of such a process is the performing of operations on sheets of paper. In application of glue lines, for example, the conveyor moves quite fast and a change in speed can cause considerable inaccuracy in the application of the glue if compensation time is applied based on an assumed speed. The results are adversely affected as well simply by relying on timing for dispensing the glue, assuming the paper will be at certain positions at certain times based on assumed conveyor speed.
Typically several settable potentiometers have been used in the operations of glue-applying machines, with these potentiometers being settable by the operator to allow for the compensation time of each glue head. However, the operator has typically had to discard several sheets, because he had to make trial and error runs of several sheets in order to correctly "tune" the potentiometers to apply and cut off electrical pulses at precisely the right times to achieve the glue pattern in the desired position. Also, in the actual run, once the timing of the glue heads is tuned, one or several more sheets may need to be thrown away because the conveyor is not yet up to ideal speed, affecting the location of the glue application.
As such, current glue dispensing techniques and systems typically assume that the conveyor, and therefore the passage of the paper or other workpiece, maintains a constant speed during the process thereby resulting in both product and time losses.